1. Field of the Invention
The present invention relates to a surface acoustic wave element that has a configuration in which a top surface of each of a piezoelectric substrate and a comb-shaped electrode is covered with an insulating film and also relates to a method of manufacturing such a surface acoustic wave element.
2. Description of the Related Art
Mobile terminals, typified by cellular phones, require a large number of parts, including a filter device and a duplexer. In recent years, to address downsizing demands, a surface acoustic wave element has been frequently used as a resonator used in such parts.
A filter device used in high frequency bands of, for example, a cellular phone, is required to have good temperature characteristics. Accordingly, to improve a negative temperature coefficient of frequency that a piezoelectric substrate included in a surface acoustic wave element has, a silicon oxide film that has a positive temperature coefficient of frequency is disposed so as to cover the top surface of each of the piezoelectric substrate and a comb-shaped electrode.
If this silicon oxide film is formed by a conventional sputtering method, the silicon oxide film may have concavity and convexity in its surface or may have a crack or a cavity within the silicon oxide film. As a result, propagation efficiency of an elastic wave decreases, and an energy loss of a resonator increases. In Japanese Unexamined Patent Application Publication No. 2005-176152, a technique of planarizing the surface of a silicon oxide film formed by a bias sputtering method and making the inside of the silicon oxide film to have uniform density is proposed.
However, the configuration disclosed in Japanese Unexamined Patent Application Publication No. 2005-176152 is problematic as described below.
When the surface of a silicon oxide film is planarized, an energy loss of a resonator is reduced, but a spurious response occurs near a resonant frequency of a principal response. Accordingly, if a filter device is configured using this surface acoustic wave element, the frequency characteristic near its pass band decreases.